Amplitude limiter circuit



June 16, 1942. 2,286,442

R. E. scHocK AMPLITUDE LIMITER CIRCUITS 2 Sheets-Sheet l Filed Dec. 6,1940 INVENTOR ATTORNEY J1me 1942 R. E. scHocK AMPLITUDE LIMITER CIRCUITS2 Sheets-Sheet 2 Filed Dec. 6, 1940 INVENTOR BIJM/ZZ: w

- ;\+TORNEY overloading of an amplifier circuit.

Patented June 16, 1942 AMPLITUDE murrnn cmcUrr Robert E. Schock,Riverhead, N. Y., assignor to Radio Corporation of America, acorporation of Delaware Application December 6, 1940, Serial No. 368,782

1 Claim.

My present invention relates to amplitude limiter circuits, and moreparticularly to limiters of the type adapted for use inconnection withphase, or frequency, modulated carrier waves.

As is well known to those skilled in the art, phase, or frequency,modulated carrier wave receivers must be provided with an amplitudelimiter at a point prior to the frequency modulation detector network.This is, necessary because the modulated carrier energy applied to thedetector must be a constant amplitude, frequency-variable signal inorder to have the detector output be a faithful reproduction of themodulation signals originally applied to the carrier wave at thetransmitter. The function of the limiter is to clip any amplitudevariation in the modulated carrier so that the limiter output is ofconstant amplitude. In most of the limiters of amplitude modulationeffects commonly used at present the limiting action depends upon the Insome cases there is used some type of fast volume control whichdemodulates the amplitude variations of the carrier. I

One of the main objects of my present invention is to provide animprovement in amplitude limiters wherein a bucking voltage is developedwhich balances out the signal to a greater or lesser extent as theamplitude of the signal rises or falls at the input terminals of thenetwork.

Still other objects of my invention are to improve generally theefliciency of amplitude limiter networks, and more particularly toprovide an amplitude limiter network for a phase, or frequency,modulated carrier wave receiver which is reliable in operation.

The novel features which I believe to be characteristic of my inventionare set forth in particularity in the appended claim; the inventionitself, however, as to both its organization and method of operationwill best be understood by reference to the following description takenin connection with the drawings in which I have indicateddiagrammatically several circuit organizations whereby my invention maybe carried into effect.

In the drawings: 1

Fig. 1 shows one arrangement of a limiter embodying my invention,

Fig. 2 shows a modification,

Fig. 3 shows another modification,

Fig, 4 graphically shows the limiter operation.

Before explaining the circuit details of the amplitude limiter networkshown .in Fig. 1, it is to be understood that the following specificexample of the use of the limiter between the intermediate frequencyamplifier and the frequency modulation detector of a frequencymodulation receiver of the superheterodyne type, is purely illustrativein nature. The present amplitude limiter is generally adapted to be usedin any situation wherein amplitude variations are desired to be removedfrom a signal. However, for the purpose of illustration, let it beassumed that the amplitude limiter network in Fig. 1 is positionedbetween the intermediate frequency amplifier stage and the frequencymodulation detector of a frequency modulation receiver of thesuperheterodyne type. Those skilled in the art are fully acquainted withthe construction of such a receiver. For example, in the presentlyassigned frequency modulation band of 43 to 50 megacycles, eachmodulated carrier wave may occupy a channel of 200 kilocycles.

Hence, the input transformer I would have its primary and secondarycircuits P and S each tuned to the center frequency of the frequencymodulated intermediate frequency energy. Such a center frequency may beof any value in a range of 2 to 4 megacycles, as, for example, 4.3. Itis to be understood, then, that there is impressed upon the resonantprimary circuit P frequency modulated carrier waves whose centerfrequency is of the operating intermediate frequency value. By virtue ofsuch effects as fading, noise impulses, and passage through priorresonant circuits, carrier amplitude variation exists at P. In otherwords, the carrier wave is not only frequecy modulated, but, also,includes amplitude modulation.

The amplitude limiter network itself comprises an electron dischargetube, say of the screen grid type, which has its signal grid connectedto the high potential side of the tuned secondary circuit S. The cathodeis connected to ground through a self-biasing resistor l3, the latterbeing shunted by an intermediate frequency by-pass condenser ll. Byestablishing the low potential end of the input coil of circuit S atground potential the signal grid of tube 2 is established at a negativebias which is equal to the voltage developed across resistor [3. Asecond electron discharge tube 4, also of the screen grid type, has itssignal grid connected to the high potential side of circuit S through apath which includes the adjustable tap H, the potentiometer resistor 6and the direct current blocking condenser 5. Signals are fed from thecircuit S to the signal grid of tube 4 through the condenser andresistor 8. The cathode of tube 4 is connected to ground through aselfbiasing resistor l5 which is shunted by the con-' denser 18 whichby-passes the amplitude modulation frequencies it is desired tosuppress. The screen grids of tubes 2 and 4 may be connected in commonto a source of positive screen grid potential, the condenserby-passing'the screen grid lead to ground for intermediate frequencycurrents. It is necessary that condenser l8 be a by-pass for theamplitude modulation frequencies it is desired to suppress, since,otherwise, the amplitude modulation voltage changes supplied to grid I!of tube 4 from resistor 8 will cause the voltage drop across resistor IIto fluctuate simultaneously in such a manner as to tend to offset thegrid voltage changes, and, therefore, tend to hold the gain of the tubeconstant. If condenser I6 is a by-pass for these amplitude modulationfrequencies then the gain of tube 4 will fluctuate in accordance withthe voltages fed to its grid from resistor 8.

The output electrodes, or plates, of tubes 2 and 4 are connected toopposite ends of the primary coil 8 of the output intermediate frequencytransformer 3. The coil 3' is shunted by condenser 8" which tunes thecoil to the 4.3 megacycle center frequency of the frequency modulatedcarrier waves. The center tap on coil 8' is connected to the source ofpositive potential required for the plates of tubes 2 and 4. Thesecondary coil 2' of transformer 3 is, also, shunted by a condenserwhich tunes the coil to the center frequency. The signal energydeveloped across the secondary circuit is applied to a frequencymodulation detector of well known type. Of course, if the receiver isnot a frequency modulation receiver, then the constant amplitude carrierwaves are transmitted to any other utilizing circuit.

Between the high potential side of circuit S and ground there isconnected a rectifying network which comprises a diode 1 arranged inseries with a load resistor 8, the latter being shunted by theintermediate frequency by-pass condenser 9. The diode 1 together withthe resistor 8 and by-pass condenser 9 constitute an amplitudemodulation detector. The resistor 8, also, provides a potentiometer bymeans of which the detected voltage developed across resistor 8 may befed to the signal grid of tube 4 through a low pass filter consisting ofcondensers I0, I I and inductance coil l2. The adjustable tap I8 isconnected to the junction of condenser l0 and coil l2.

Consider, now, the operation of the limiter circuit. Assume, forexample, that potentiometer tap H has been adjusted so that only afraction of the signal voltage fed to the signal grid of tube 2 is fedto the signal grid of tube 4. Assuming that the gains of the two tubes 2and 4 are approximately the same, only a fraction of the signal energywill be balanced out in the primary winding 3' by virtue of thepush-pull action of tubes 2 and 4. The amount of signal energybalancedout in the primary winding 3 under these circumstances dependson the ratio of signal from tube 2 to that of tube 4. The amplitudemodulation detector (elements 1, 8 and 9) functions to furnish a biasingvoltage to tube 4 of such a character that its gain will be modulatedsimul-- taneously with the amplitude variations of the modulated'carrier waves applied to the transformer I.

Therefore, the ratio of the outputs of tubes 2 level. Then, theamplitude of the wave voltage developed at the output electrode of tube2 increases thus tending to increase the output of transformer 3.However, this increase or amplitude also occurs at the diode detector 1,and hence, the rectified current through resistor 8 increases. Thisresults in an increase in the voltage developed across resistor 8, andthe cathode end 01' resistor 8 becomes increasingly positive inpotential.

By virtue of the tap l8 this increased positive direct current voltageis fed through the low pass filter to the signal grid of tube 4 therebyincreasing the gain 01 the latter. As a result or the increase in gainof tube 4 the amount of bucking voltage in plate winding 8 will balanceout the amplitude increase-due to tube 2. It isto be understood that theadjustable taps l8 and II of potentiometers 8 and 8 respectively are setso as to provide the proper proportioning of signal levels for tubes 2and 4 in the absence of amplitude variation, and, also, to providesufllclent positive voltage for the grid of tube 4, when amplitudeincrease occurs, to produce bucking voltage in the winding 3'. It willnow be seen that there has been provided as an amplitude limiter devicean electron discharge tube which has impressed upon its input terminalsthe frequencyvariable carrier waves, and a second electron dischargetube being arranged so as to have applied to its input terminals apredetermined fraction of the frequency-variable carrier waves.

The output circuits of the two devices are arranged in phase oppositionrelation'so that there exists a normal frequency-variable carriervoltage output from both devices as long as the amplitude of the wavesat the input terminals of the limiter are of substantially constantamplitude. However, upon a variation in the amplitude level of thecarrier waves there is derived from the carrier waves a control voltagewhich is applied to the second of the devices in a sense to vary theoutput of the second device so that the predetermined output amplitudelevel is maintained at the limiter output terminals. It will, also, beobserved that the normal gain of tube 4 depends not only upon the biasdeveloped across resistor l5, but also upon the normal direct currentvoltage developed across the lower portion or load resistor 8. Ofcourse, the tap l8 will be so adjusted that a predetermined magnitude ofcontrol voltage developed across resistor 8 is applied to the signalgrid of tube 4 thereby to establish the normal gain of the tube.

Fig. 4 shows an ideal limiter characteristic with limiter input as theabscissa, and limiter output as the ordinate. It may be seen from thiscurve that any amplitude variation of the signal input which causes itto dip below point A will cause the output to dip., Therefore, it isessential that the limiter be operated at some input level such as Bwhere the input may vary between A and C in amplitudewithout eflecting achange in the output. This applies as well to the limiter oi thisapplication, and it it is operated at a point corresponding to point Bit reacts to eliminate amplitude dips as well as peaks.

The circuit arrangement in Fig. 2 differs from that shown in Fig. 1 incertain of the circuit details. In this case the input transformer Ifeeds the modulated carrier waves to the signal grid of the tube 2, andthe latter in turn feeds its output voltage to the tuned transformer 3.The diode rectifier 1 is again arranged as in the case of Fig. 1 tofunction as the detector of the amplitude modulation of the carrierwaves. The control voltage developed across the selected portion ofresistor 8 is transmitted to the signal grid of the compensating, orbucking, tube 4 through the path including the tap 18, the coil 58 andthe lower portion of resistor 6. The low pass filter in the present caseconsists of the condensers 56 and 51 as well as coil 58. In thismodification the signal grid of tube 4 is fed with modulated carrierwave voltage from the plate circuit of tube 2 through the direct currentblocking condenser 60 and the upper portion of resistor 6. The plate oftube 4 feeds its output voltage to the tuned primary circuit oftransformer 3.

In operation, the modulated carrier waves are amplified by tube 2, andfed to the transformer 3. The modulated carrier voltage output of tube 4is in phase opposition to the modulated carrier voltage output of tube 2in their common plate output circuit which is the tuned primary circuitof transformer 3. If the carrier amplitude at the limiter inputterminals increases, the bias voltage developed across resistor 8increases in a positive polarity sense. This control voltage is appliedto the signal grid of tube 4 thereby increasing the gain of the latter.As explained previously, the increase in gain of tube 4 occurssimultaneously with the carrier amplitude increase at the inputterminals of tube 2. Consequently, there is developed in the commonoutput circuit of tubes 2 and 4 an increased amount of bucking carriervoltage due to tube 4. Hence, the net result, in so far as the primarycircuit is concerned, is that no change of carrier amplitude occurs atthe secondary circuit of transformer 3. It is to be understood that inthis case, also, the taps l8 and H are adjusted to produce thisbalancing effect.

The plates of tubes 2 and 4 are connected in parallel, but the phase ofthe signal received by the grid of tube 4 is 180 degrees displaced fromthat received by the grid of tube 2 since it has passed through tube 2before reaching tube 4 tion as a volume limiter the condenser 9 wouldand has received a phase reversal in tube 2.

transformer of Fig. 1, and, therefore, I have shown in Fig. 3 a circuitwhich keeps this advantage but eliminates the inverse feedback of tube4, Fig. 2, by using a phase inverter tube (designated as 6|) to feedtube 4. In other respects Fig. 3 is the same as Fig. 2.

have to be a by-pass for voice frequencies but not for syllabicfrequencies, so that the detector 1 becomes a detector of syllabicfrequencies, or volume dips and peaks, and passes these detected dipsand peaks on to the grid of tube 4. This would vary the gain of tube 4so that the bucking action of its plate output on the plate output of,tube 2 would effect a more or less constant volume in the output asdesired. Condenser IS in this case would have to be a by-pass for thesyllabic, or volume peak and dip, frequencies. Also, the transformers Iand 3 would have to be audio transformers since its use as a volumelimiter, or volume compressor, would be at audio frequencies. It wouldbe effective on volume dips as well as volume peaks.

While I have indicated and described several systems for carrying myinvention into effect, it will be apparent to one skilled in the artthat my invention is by no means limited to the particular organizationsshown and described, but that many modifications may be made withoutdeparting from the scope of my invention, as set forth in the appendedclaim.

What I claim is:

In a frequency modulated carrier wave transmission system of the typecomprising an amplifler tube having input and output electrodes, aresonant input circuit coupled to the input electrodes and tuned to thecenter frequency of applied waves and having a pass band such as to passall the frequency deviations of said center frequency, a resonant outputcircuit coupled to said output electrodes and having a frequencyresponse characteristic similar to said input circuit; the improvementcomprising means for reducing amplitude modulation of said carrier wave,said means comprising an electron discharge device provided with inputand output electrodes, means coupling said input circuit to said lastinput electrodes to apply a predetermined fraction of the modulated waveenergy thereto, means connecting the last output electrodes to saidoutput circuit to provide in the latter the differential output of saidamplifier tube and said device, and means connected to said inputcircuit, constructed and arranged to be responsive to said amplitudemodulation, for automatically varying the gain of said device in a senseto insure said differential output being substantially free of saidamplitude modulation, said coupling means comprising an electrondischarge tube acting as a phase inverter, and the output electrodes ofsaid amplifier tube and device being connected in parallel to saidoutput circuit.

' ROBERT E. SCHOCK.

